Magnesium air batteries, i.e. batteries comprising a negative electrode (anode) comprising or consisting of magnesium or of a magnesium alloy, have been attracting an increased attention as magnesium possesses higher volumetric capacities than e.g. lithium, i.e., 3832 mAh cm−3 for magnesium vs. 2061 mAh cm−3 for lithium. Compared to nowadays commonly used lithium-ion batteries, magnesium air batteries are possibly about twice as powerful because of the high volumetric capacities. Additionally, the energy per mass is significantly higher for magnesium air batteries compared to lithium-ion batteries. More importantly, however, the electrochemical processes related to its reversible plating/stripping have demonstrated the absence of dendrites formation which has thus far alleviated safety concerns related to employing it as a negative electrode in batteries. Finally, magnesium may also provide an opportunity for battery cost reductions due to its natural abundance in the earth crust.
However, several technical challenges still hamper the commercialization of magnesium air batteries. In fact, the absence of practical electrolytes and cathodes has confined demonstrations of rechargeable magnesium batteries to research laboratories (cf. R. Mohtadi, F. Mizuno “Magnesium batteries: Current state of the art, issues and future perspectives”, Beilstein J. Nanotechnol. 2014, 5, 1291-1211). That is, low gravimetric energy densities in the order of few hundreds watt hour per kilogram and a limited shown durability coupled with very sluggish kinetics make magnesium batteries currently far from being practical.
When discussing the magnesium metal, the nature of its interaction with the electrolyte has been recognized to represent an important and complex topic. That is, interfaces formed on the metal resulting from metal-electrolyte interaction have a direct impact on electro-chemical properties related to the dissolution and plating of the metal, i.e., discharge and theoretical charge of the battery. It is well established that the formation of a surface layer on magnesium anodes as a result of metal-electrolyte chemical/electrochemical interaction is detrimental for reversible magnesium deposition, as it blocks the transport of the magnesium ions thereby preventing reversible electrochemical dissolution and plating from taking place (cf. R. Mohtadi, F. Mizuno “Magnesium batteries: Current state of the art, issues and future perspectives”, Beilstein J. Nanotechnol. 2014, 5, 1291-1211).
In particular, the use of aqueous electrolytes in magnesium air batteries may lead to problems such as enhanced anode corrosion, voltage drop, electrolyte instability and, in particular, self-corrosion processes leading to uncontrollable degradation and destruction of the magnesium anode. All of these processes lead to a reduction of overall battery performance and lifetime.
Until now, there have only been a few attempts to enhance the battery performance by controlling the interaction between the magnesium anode and the aqueous electrolyte and these attempts show a number of disadvantages such as overcharging, capacity fading, non-conductive passivation of the anode surface or voltage drop over time.
For example, in order to improve the performance of a magnesium battery, Wang et al. (cf. N. Wang et al., Research progress of magnesium anodes and their applications in chemical power sources, T Nonferr Metal Soc., 24 (2014) 2427-2439) applied different magnesium alloys as anode material (e.g. AZ series, AM series, Mg—Li) which lead to the possibility of tuning the microstructure of the anode and to a higher flexibility. However, the use of said alloys leads to undesired secondary effects such as unwanted reactions and to a voltage drop over time.
Therefore, there is still a need for new electrolyte compositions for magnesium air batteries operating in aqueous electrolyte environments, especially there is the need to provide efficient means for enhancing the overall performance of magnesium air batteries comprising an aqueous electrolyte.